Add Octopus Deploy Platform Hub knowledge base

New files:
- PLATFORMHUB.md: index for Platform Hub guidance
- platformhub/architecture.md: Git→PlatformHub→Projects model
- platformhub/ocl-syntax.md: complete OCL reference for templates
- platformhub/process-template-patterns.md: 5 proven patterns
- platformhub/gotchas.md: every error hit and how to fix it
- platformhub/api-reference.md: API vs UI capabilities
- best-practices/octopus-process-templates.md: consolidated best practices

Learned from building PlatformHub-Demo (30 microservices, 3 clouds).
Key discoveries: step templates are space-scoped (can't cross-reference),
worker_pool parameter is mandatory, publishing/sharing is UI-only.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

BESTPRACTICES.md

@@ -20,3 +20,5 @@ Generalised best practices extracted from real project work. Each topic file is
 - [Test-Driven Development](best-practices/test-driven-development.md) — Edge case discovery, property-based testing, mutation testing, AI agent testing patterns, test architecture
 - [Networking & Infrastructure](best-practices/networking.md) — nftables safety, systemd socket activation, Docker forwarding, TLS SNI vs Host header, wildcard certs
 - [Docker UID Matching](best-practices/docker-uid-matching.md) — UID wrapper entrypoint for mounted volumes, gosu pattern, when to use vs K8s securityContext
+- [Octopus Process Templates](best-practices/octopus-process-templates.md) — OCL syntax, step template references, channel scoping, parameters, versioning, Platform Hub patterns
+- [Platform Hub Knowledge Base](PLATFORMHUB.md) — Comprehensive guide: architecture, OCL syntax, template patterns, gotchas, API reference